Inkjet printing mechanisms, e.g., printers, plotters, photocopiers, facsimile machines, etc., typically implement inkjet cartridges, often called “pens” to shoot drops of ink onto a sheet of print media, e.g., paper, fabric, textile, and the like. Some inkjet printing mechanisms carry an ink cartridge with an entire supply of the ink back-and-forth across the sheet. Other inkjet print mechanisms, known as “off-axis” systems, propel only a small ink supply with the printhead carriage across the print zone, and store the main ink supply in a stationary reservoir, which is located off-axis from the path of the printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis reservoir to the printhead cartridge.
Inkjet printing mechanisms typically comprise a plurality of inkjet pens of various colors. For example, a typical inkjet printer/plotter may comprise four pens, one that prints black ink, and three that print colored inks, e.g., magenta, cyan and yellow. The colors from the three color pens are typically mixed to obtain any particular color.
The pens are typically mounted in stalls within an assembly that is mounted on the carriage assembly of the printing mechanism. The carriage assembly generally positions the inkjet pens and typically holds the circuitry required for interface with components, e.g., firing resistors, piezoelectric elements, and the like, that operate the inkjet pens.
Color printing and plotting generally requires that inks from each pen be precisely applied to the print media. This requires precise alignment of the carriage assembly. However, mechanical misalignment of the pens in conventional inkjet printing mechanisms typically results in offsets in the direction of carriage travel and offsets in the direction of print media travel. This misalignment of the print carriage assembly manifests as a misregistration of the images applied by the pens. In addition, other misalignments may arise due to the speed of the carriage, the curvature of the platen and/or spray from the nozzles, and the like. Furthermore, the misalignments may arise from difficulties that may arise during the manufacture of the pens, such as imperfect nozzle shape and/or placement.
One manner in which conventional printing mechanisms attempt to overcome the problems associated with the carriage assembly misalignments is through implementation of optical systems designed to perform detections on a test strip. More specifically, conventional printing mechanisms may include optical detectors mounted on the carriage assembly for detecting test strips printed by each of the pens. The optical detectors typically consist of one or more light emitting diodes (LED), typically of different colors, that illuminate an area or surface of the media and an optical sensor that receives the signal reflected from the media. Although conventional optical systems have been found to be effective in detecting relative small test strips and certain colors, they also have certain drawbacks and disadvantages.
For example, conventional optical systems have a substantially limited field of view (e.g., about 1270×1270 μm). Therefore, detection of relatively wide areas with conventional optical systems require performance of several scans, thereby increasing the time required to perform the detections. In addition, conventional optical systems are often limited to sensing colors in the bands of the color spectrum corresponding to the LEDs implemented in the optical systems. One consequence of which is that some of the printed colors may not be accurately detected by the optical systems. Thus, although conventional optical systems have been relatively effective in detecting test strips formed by pens having relatively small swath heights (i.e., pens having a relatively small number of nozzles), conventional optical systems are ill-equipped to detect test strips formed by today's printing mechanisms that utilize pens having a much larger number of nozzles.